Cathode ray tube



April 28, 1959 A. T. RAczYNsKx 2,884,551

CATHODE RAY TUBE;

Filed June 29. 1955 2 Sheets-Sheet 1.

WITNESSES INVENTOR Andrew T. Ruczynski.

pril 28, 1959 A. T. RAczYNsKl cATH'oDE RAY TUBE 2 Sheets-Sheet 2 Filed June 29. 1955 Relative Flux Density Pole Piece Spacing (Inches) $22.3 3.30am o :ramo Eo: ...0:23a Bavo *cnw 'that deactivated area.

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United States Patent O CATI-IODE RAY TUBE Andrew T. Raczynski, Horseheads, N.Y., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 29, 1955, Serial No. 518,871

3 Claims. (Cl. 313-76) This invention relates to apparatus for producing an velectron beam and, more particularly, to the structure activated area of uorescent material which is referred to -as ion burn or blemish absorbs a high proportion of the electron beam resulting in a small light output from It is the removal of the undesirable ions from the electron beam to which this invention is directed.

The mass of the ion is greater than the electron While the charge is essentially the same. The negative ion will be focused and deflected essentially along the same path as the electron in an electrostatic eld while a magnetic iield sufficient to focus the electrons to the extent necessary in a cathode nay tube will have little effect on the negative ions. It is, therefore, seen that the ions would bombard the entire screen with electrostatically deflected type cathode ray tubes and essentially only one spot with electromagnetically deected type cathode ray tubes. It has been found that an ordinary cathode ray tube with magnetic deflection may develop an ion blemish in as little as one hour of operation. The use of electrostatic deflection results in reduced ion bombardment over the ventire face of the cathode ray tube yand the life of the tube ismuch longer. However, over a short period of operation the electrostatic deflection type tube will have reduced light output over the entire screen.

There are several methods of reducing the objectionable ion bombardment by choice of suitable gettering material, fluorescent screen material and materials used in the electron gun structure. Another reduction method ythat is also used is metallic backing material on the is deflected by a single magnetic field referred to as the ion trap magnet onto and along the longitudinal axis of vthe tube neck. These type structures which may be referred to as bent gun structure require that the'individual parts of the electron gun are set at angles with respect to other parts resulting in Weak mechanical structures.

It is also obvious that such structures are more ditiicult electron gun parts are as- Some structures which are Patented Apr. 28, 1959 ffice but it is necessary that the gun be tilted within the tube neck and also that the diaphragm nearest the iluorescent screen be tilted with respect to the anode cylinder or that the aperture inthe diaphragm be located off center. With both the tilted guns and the bent guns the neck of the tube must be of a larger diameter than if the entire gun Was coaxial with the tube neck. A coaxial electron gun structure is `described in U.S. Patent 2,496,127, but the structure requires two ion traps which overbalances the advantages of the coaxial structure. The gun described in the above mentioned U.S. patent utilizes a slashed electrostatic field to deflect both the electrons and the ions generated by the cathode of the electron gun off the tube `axis and then -a rst magnetic field is utilized to return the electron beam back to the axis. Since the electron beam returned to the axis by the lirst magnetic field approaches at an angle to the axis, it is necessary that a second magnetic field be utilized to align the electron beam along the main axis of the tube. The ions are substantially not elected by the magnetic elds and are trapped Within the gun.

It is, accordingly, an object of my invention to provide an improved electron gun structure capable of providing a substantially ion free electron beam.

It is another object to provide an electron gun in which the parts are positioned coaxial Within the tube neck and requiring only one ion trap magnet.

It is another object to provide an economical gun structure with the ion trap for utilization within small diameter neck cathode ray tubes.

It is another Iobject to provide a structure in which the electron beam remains substantially on the longitudinal axis of the electron gun structure while the ions are reected olf the axis.

These and other objects are eected by my invention as will be apparent from the following description taken in accordance with the accompanying drawings throughout which like reference characters indicate like parts and in which:

Fig. l is a side elevational view showing the relationship of the cathode ray gun embodying the principles of my invention to the other structural elements of a cathode ray tube;

Fig. 2 is a side elevational View partly in section of a cathode ray gun embodying the principles of my invention;

Fig. 3 is a transverse view of the screen grid according to my invention;

Fig. 4 is a cross-sectional view` of the screen grid; and

Figs. 5, 6 and 7 are graphs for purpose of explanation.

Referring in detail to Fig. l, there is shown a cathode ray tube employing my invention. The tube is comprised of an envelope 10 having a tubular neck portion 12, a flared bulb portion 14 and a face plate 16. The face plate 16 of the envelope 10 has a suitable fluorescent coating 18 on the inside surface thereof, and the flared bulb portion 14 has `a conductive coating 20 of a material such as graphite. An electron gun 22 is mounted on the inside of the neck portion 14 of the envelope 10.

The electron gun 22 is comprised of an indirectly heated cathode 30, a control grid or electrode 40, a screen grid or electrode 50, a first anode 60, a focusing electrode 70, and a second anode 80, in the order named.l The coating 20 on the interior of the flared bulb portion serves as the third anode of the cathode ray tube, and a suitable voltage is supplied at terminal 26. rThe structure described and shown herein is of the type known as an electrostatic focus type cathode ray gun. Positioned on lthe exterior of the neck portion 12 of the envelope 10 is electrostatic or electromagnetic delieetion system represented by the coil or yoke 27.

Referring in detail to Fig. 2, the control grid 40 is comprised of a tubular member or skirt 42 coaxial with the tube neck axis 11 with a diaphragm or stop 44 positioned therein in which a small aperture 46 is centrally located on the axis 11 of the tube neck. In the specific embodiment shown, the diaphragm 44 is located at the end of the cylindrical or tubular member 42 nearest the screen 18. The plane of the diaphragm 44 is perpendicular to the axis 11 of the tube neck. Positioned within the control grid structure 40 is the cathode 30 Icomprised of a tubular member 32 of smaller diameter than the control grid skirt 42 with its logitudinal axis lying on the axis 11. The end of the tubular member 32 nearest the diaphragm 44 of the control grid 40 is closed and a coating 34 of electron emissive material is placed on the exterior surface. The tubular member 32 is supported in the grid skirt 42 by means of a ceramic collar 36. A heater lament 38 is provided within the tubular member 32 for controlling the temperature of the electron emissive coating 34.

The screen grid or electrode 50 is comprised of a tubular skirt or cylindrical member 52 spaced along the axis 11 and adjacent to the diaphragm end of the control grid 40. The screen skirt 52 has a diaphragm 54 positioned therein, and in the specific example closes the end of the cylinder 52 adjacent to the control grid 40 and is perpendicular to the main axis 11. In a similar manner to the control grid diaphragm 44, an aperture 56 is placed at the center of the diaphragm 54. The unclosed end or rim 58 of the screen skirt 52 may be rolled outward as shown in Fig. 2 to eliminate possibilities of distortion of electrostatic fields due to sharp edges and burs on the material. The plane of the unclosed end or rim 58 of the cylinder 52 is slanted at an angle of 10 to 13 with respect to a plane perpendicular to the axis 11.

The screen grid 50, as generally described above, is more completely described in a copending application, Serial No. 374,240, tiled August 14, 1953, now Patent No. 2,773,212, entitled Electron Gun, by James A. Hall, and assigned to the same assignee. In my invention, it is necessary to incorporate two pole piece members 51 and j 53 adjacent to the apertured diaphragm 54 of the screen grid 50. The pole pieces 51 and 53 are made of high permeability material such as soft iron. The pole pieces 51 and 53 may be inserted through openings 55 and 57 provided in the skirt or cylindrical Wall 52 adjacent to the closed end or it may be desirable to attach the pole pieces 51 and 53 on the opposite side of the diaphragm 54. In the specific example shown, the pole pieces 51 and 53 are inserted within the interior of the screen grid 50 from opposite sides of the aperture 56 and positioned at equal distances from the aperture 56. The spacing of the pole pieces from each other is of the order of .055 inch and extend out adjacent to the neck portion 12 of the envelope to offer minimum air gap to external magnet 21. The pole pieces 51 and 53 enter from opposite sides of the screen grid 50 at a point where the length of the cylinder 52 is equal to the mean length.

The rst anode 60 is comprised of a" cylindrical or skirt member 62 spaced along the axis 11 and adjacent to the slanted open end of the screen grid 50. The rst anode 60 is open at the end adjacent the screen grid 50 and the rim 68 is also slanted so that the plane of the rim 68 is parallel to the plane of the rim 58 of the screen grid 50 and inclined to a plane perpendicular to the axis 11 by about 10 to 13. The other end of the first anode skirt 62 is closed by a diaphragm 64 perpendicular to the axis 11 also having a centrally located aperture 66 therein. The second anode 80 is comprised of a cylindrical or skirt portion 82 positioned along the axis 11 at a distance greater than the other members of the gun from the first anode 60. The end of the skirt 82 adja- Cent the rst anode 60 is closed by a diaphragm 84 perpendicular to the axis 11 and having a centrally located aperture 86 therein. The opposite end of the second anode skirt 82 also has a diaphragm 83 therein perpendicular to the axis 11 with a large central aperture 85 therein. Integral with the diaphragm 83 is a flange 88 extending outwardly from the second anode 80 to which flexible spring members 90 are attached which position the electron gun 22 within the neck 12 and also make electrical contact with the Aquadag coating 20 on the flared portion 14 of the envelope 10. The first and second anodes 60 and 80 are connected together electrically and are supplied with voltage from the Aquadag coating 20 by means of the flexible spring members 90. A sleeve or focusing electrode 70 surrounds the space between the first and second anodes 60 and 80, and is coaxial with the axis 11 and of larger diameter than the rst and second anode cylinders 62 and 82. The control grid 40, the screen grid 50, the rst anode 60, focusing electrode 70 and second anode 80 may be supported by providing radially jetting anchor pins on the cylindrical surfaces thereof. The anchor pins 100 are embedded within longitudinal glass support rods 102 extending along substantially the entire length of the gun structure. Suitable voltages are supplied to the elements of the electron gun 22 by means of leads (not shown) extending through the button stem provided in the end of the tube neck 12. A table is given below of typical dimensions of a specific embodiment shown in Fig. 1 with representative voltages of each member. The relative dimensions of these electrodes are expressed as ratios to the diameter of the control grid cylinder 42.

Cylinder Diameter Length Voltage (Outside) V- 0.72 0. (Inside) 1- 0.87 A -30. (Inside) 1- 0.71 (maximum length) +275 to 500 v. (Inside) 1 2.57 (maximum length). +10 to 18 kv. 1 1.02 l0 to 18 kv.

Aperture: Inside diameter 46 .062 56 .072 76 .200 86 .240

Gap-between electrodes along axis 1I 40-50 0.045 50-60 .2 60-80 .75

In the operation of the electron gun structure 22, the electrons as well as the negative ions are emitted from the electron emissive surface 34 of the cathode 30 and the electrostatic lield between the control grid 40 and the screen grid 50 accelerates and forms ya lens of short focal length bringing the electrons and ions to what is known as a cross-over point between the two diaphragms 44 and 54. The control grid 40 controls essentially the number of electrons passing through the aperture 46 in the diaphragm 44, and thereby controls the intensity of the electron beam. The electron and ion beam thus formed passes through the aperture 56 in the diaphragm 54 of the screen grid 50. The electrostatic eld or lens set up between the screen grid and the rst anode in effect acts on the beam at the time of entering the aperture 56 in the diaphragm 54 of the screen grid 50 and acts to focus and deflect the electron and ion beam entering the teld. Since the adjacent ends of the screen grid 50 and the rst anode 60 lie in parallel planes tilted to a plane perpendicular to the axis 11, the effect is that a bending electrostatic iield is impressed on the electrons and ions within the beam just after cross-over. Since the electrons and ions are moving slowly at this point, due to the low acceleration voltages, then the electron and ion beam is bent sharply downward at this point. The magnetic field between the pole pieces 51 and 53 is also concentrated at the point just past cross-over so that a magnetic field is set up with the force directed upward onthe electron and ion beam or opposing the electrostatic iield force. Since as previously stated, the de- ,flection of particles in the magnetic field depends on the mass of the particle involved, the field can be adjusted to compensate for the electrostatic iield as to the electrons, while little eiect will be had by the magnetic field on the ions which are of much lgreater mass. The ion beam will thus follow substantially the deflection path 4impressed thereon'by t-he electrostatic eld set up between vthe screen grid50 andthe rst anode 60, and will strike the Wall 62 of the iirst anode 60 while the electron beam will remain substantially on the axis 11 and pass through the aperture 66 of the first anode 60. The importance of the structure disclosed herein is the need for the concentration of the magnetic field to a narrow region by the pole pieces 51 and 53 to substantially overcome the electrostatic field set up between the screen grid 50 and the first anode 60 as to the electron beam. It is also important to realize that the positioning of the pole pieces 51 and 53 with respect to the aperture 56 and the size of the pole pieces 51 and 53 are of great importance.

In Fig. 5 there is shown a plot of the magnetic flux density versus distance along axis 11 (Fig. 2) with different arrangement of pole pieces. The graph shows that the maximum concentrated intensity is obtained by utilizing pole pieces filling as much of the gap between the electron beam and the magnet pole plates as possible represented by curve 110. When smaller pole pieces were used it was found that location of such pole pieces closed to the beam (curve 111) was more desirable than location close to the magnet (curve 112). The curve 113 represents the lield distribution where no pole pieces were used. In Fig. 7, using a given length and width of pole piece as represented by curve 110 of Fig. 5, various thicknesses of pole pieces were used. It was found that the greatest concentration of llux density was obtained with the pole pieces physically thin. The curves 114, 115 and 116 of Fig. 6 were obtained with pole pieces having a thickness of .O18 inch, .026 inch and .036 inch, respectively.

It is known that the angle of exit of the electron beam from the gun determines the undeected spot location on the tube screen. An electron beam leaving the gun on axis 11 will have a deviation of zero from the center of the face plate. Therefore, emergence of the beam along the gun axis is evidence that the electrostatic and magnetic deflections in the ion trap structure neutralize each other, and the electron beam remains essentially on the axis 11 (Fig. 2) throughout its travel through the electron gun structure and the remainder of the distance to the screen. The results of tests shown in Fig. 7 indicate how the optimum spacing for a given pole piece thickness may be readily found. The curve shown in Fig. 7 is a plot of pole piece spacing versus distance of the undeilected electron spot from the geometric center of the tube face. The line 117 is plotted for pole pieces of a thickness of .018 inch and line 118 for thickness of .026 inch. However, it was found that pole pieces which come too close to the electron beam cause some aberration of the electron beam. Por example, with the aperture in the diaphragm of the screen grid having a diameter of .036 inch, a spacing of .036 inch between pole pieces of a thickness of .026 inch caused substantial aberration of the beam. It was found that with a .036 aperture and pole pieces of .018 inch thickness, the curves of Fig. 7 predict a spacing of .055 inch and models constructed along these lines were found to have satisfactory spot shape and centering.

While I have shown my invention in several forms, it will be obvious to those skilled in the art that it is not so limited, but is susceptible of various other changes and fus modifications without departing from thereof.

I claim as my invention:

l. A cathode ray tube comprising anenvelope having al tubular neck portion, an electron gun structure coaxially positioned within said neck portion, a target screen spaced from said electron gun and positioned substantially perpendicular to the axis of said neck portion, said electron gun comprised of at least a cathode, a control grid, a screen grid and an anode, said control grid comprised of a diaphragm having an aperture centrally located therein and positioned adjacent said cathode for focussing a mixed electron and ion beam generated by said cathode to a crossover point on said axis, said screen grid positioned intermediate said control grid and said anode, said screen grid and said anode having a tubular portion and having their adjacent ends lying in planes parallel to each other and tilted with respect to said axis for impressing a tilted electrostatic eld on said mixed beam adjacent said crossover point, said screen grid having a transverse diaphragm therein, said diaphragm having an aperture centrally located therein and positioned on said axis, a magnetic field impressed on said beam by means of a magnet and pole plates positioned on the exterior of said neck, and iield concentrating means positioned within said neck and extending from a point near said pole plates to a point near the aperture in said screen grid to provide a narrow iield region near said crossover point to counteract the electrostatic deilection force on said electrons.

2. A cathode ray tube comprising an envelope having a tubular neck portion, an electron gun structure coaxially positioned within said neck portion, a target screen spaced from said electron gun and positioned substantially perpendicular to the axis of said neck portion, said electron gun comprised of at least a cathode, a control grid, a screen grid and an anode, said control grid comprised of a diaphragm having an aperture centrally located therein and positioned adjacent said cathode for focussing a mixed electron and ion beam generated by said cathode to a crossover point on said axis, said screen grid positioned intermediate said control grid and said anode, said screen grid and said anode having a tubular portion and having their adjacent ends lying in planes parallel to each other and tilted with respect to said axis for impressing a tilted electrostatic ield on said mixed beam adjacent said crossover point, said screen grid having a transverse diaphragm therein, said diaphragm having an aperture centrally located therein and positioned on said axis, a magnetic eld impressed on said beam by means of a magnet and pole plates positioned on the exterior of said neck, and concentrating means positioned within the region between said pole plates and said aperture in said screen grid for concentrating said electromagnetic eld to a narrow region near said crossover point to counteract the electrostatic deflection force on said electrons, said concentrating means comprised of a plurality of metal pole pieces positioned transverse to said axis and adjacent said diaphragm in said control grid and substantially filling the region between a point near the aperture in said diaphragm in said screen grid to a point near said neck portion.

3. A cathode ray tube comprising an envelope having a tubular neck portion, an electron gun structure coaxially positioned within said neck portion, a target screen spaced from said electron gun and positioned substantially perpendicular to the axis of said neck portion, said electron gun comprised of at least a cathode, a control grid, a screen grid and an anode, said control grid comprised of a diaphragm having an aperture centrally located therein and positioned adjacent said cathode for focussing a mixed electron and ion beam generated by said cathode to a crossover point on said axis, said screen grid positioned intermediate said control grid and said the spint and cope 7 anode, said screen grid and said anode having a tubular portion and having their adjacent ends lying in planes parallel to each other and tilted with respect to said axis Afor impressing la tilted electrostatic iield on said mixed beam adjacent said crossover point, said screen grid having a transverse diaphragm therein, said diaphragm having an aperture centrally located therein and positioned on said axis, a magnetic eld impressed on said beam by means of a magnet and pole plates positioned on the exterior of said neck, and concentrating means comprised of two diametrically opposed pole pieces transverse to said axis and positioned adjacent said crossover point and substantially filling a region between the aperture in said diaphragm in said screen grid and said pole plates.

References Cited in the le of this patent UNITED STATES PATENTS 

